I think you’re doomed this time. I can’t even find support for CRAM-MD5 in the
Net::POP3 class which Discourse uses, let alone any way to configure it within Discourse. As an added bonus, there don’t appear to be a thundering herd of alternate POP3 libraries for Ruby which we could switch to to provide CRAM-MD5 support.
Short of you doing a lot of coding to add CRAM-MD5 support to
Net::POP3, and providing a patch to Discourse to add a tickbox to use that support, I think you’re out of luck here.
Alternately, do you have the ability to forward e-mails sent to your incoming address(es) to another MTA? I’m on the hook for a blog post (and associated docker container) about how to do direct-delivery incoming e-mail (obviating the need for POP3 polling at all); if that would work for you, I’ll get that post out ASAP and you can switch to that.